This invention relates to the preparation of high purity alumina and, more particularly, to the preparation of such alumina which is suitable for being isostatically pressed into tubular form and then sintered to translucent light-transmitting tubular form to a density which closely approaches theoretical.
U.S. Pat. No. 3,026,210, dated Mar. 20, 1962 to Coble discloses an alumina polycrystalline body which includes a small amount of magnesia and which has a density which closely approaches theoretical. This material is highly transmissive for visible radiations and it is particularly adapted to be formed into sintered tubular configuration for use as an envelope for discharge devices. Such an application is disclosed in U.S. Pat. No. 4,150,317, dated Apr. 17, 1979 to Laska.
U.S. Pat. No. 3,941,719, dated Mar. 2, 1976 to Yoldas, one of the present applicants, discloses the preparation of a clear hydrolyzed aluminum alkoxide sol which can be dried and pyrolyzed to form a pure alumina which is useful as a catalyst, absorbent and desiccant. The intermediate product, which may be a sol or a gel can also be used to coat substrates. A related disclosure is set forth in The American Ceramic Society Bulletin, Vol. 54, No. 3, March, 1975, pp. 289-290, article by B. E. Yoldas.
Heretofore, very finely divided alumina has been spray dried prior to isostatic pressing, and this has permitted a higher bulk density to be obtained in the pressed and unsintered compact. Spray drying apparatus is readily available commercially and a typical spray drying apparatus for use with finely divided materials is schematically shown in U.S. Pat. No. 3,023,339, dated Feb. 27, 1962 to Vodoklys. Heretofore, finely divided aluminas suitable for isostatic pressing have been prepared by a variety of processes and a typical preparation process involves the calcination of a hydrated inorganic aluminum salt, such as an alum. The calcination temperature is sufficiently high that the salt is decomposed, which voids the water of crystallization and other gaseous decomposition products, such as SO.sub.3 in the case of the sulfate component, leaving a fine particulate powder, substantially all of which is in the high temperature crystalline form of alumina (alpha).